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Small intestine
Core Procedures
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Small intestinal resection and anastomosis
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Small intestinal bypass
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Stricturoplasty
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Ileostomy creation and reversal
Surgery of the small intestine was rarely performed successfully prior to the development of anaesthetics and antisepsis. The advances of Bigelow and Lister allowed operations on the small intestine to become an essential part of the practice of the early surgeons who managed wartime injuries, treated colic, and were faced with common diseases such as appendicitis. In over 120 years of intestinal surgery, generations of surgeons have developed and refined techniques for small intestinal resection and anastomoses, many of which have not changed substantially over the past three decades, despite the advent of staplers and vessel-sealing devices. When textbooks that describe small bowel resections for different disease processes are reviewed, their reports and commentaries are strikingly similar in their description of the techniques they used and the commonly encountered pitfalls.
The small intestine is the primary site within the gastrointestinal tract for the absorption and digestion of nutrients, and therefore surgery performed on the small intestine, regardless of the indication, must first and foremost aim to preserve intestinal length. Surgery may also have an impact on the role of the small intestine as an important immunological and endocrine interface, despite the fact that this role is much less well understood. Ultimately, the guiding principle that underlies all small intestinal operations is to preserve intestinal length and continuity. The indications for surgery on the small intestine are numerous and include procedures for treatment of neoplasms, inflammatory diseases, infections and perforations, as well as traumatic injuries. However, in all of these cases, the affected intestinal segment is typically resected and gut continuity is restored by anastomosing the two healthy ends together. The remarkable redundancy of the small intestine, as well as its resilience to insult, has allowed generations of surgeons to teach themselves, and their trainees, the cautious practice of surgery.
Embryology
The embryological origin of the small intestine is complex, as is reflected by the multiplicity of cell types derived from the ectoderm, mesoderm and endoderm that comprise its absorptive, immunological and endocrine functions. The alimentary tube forms at 4–8 weeks of gestation, when its luminal and serosal surfaces develop. The developing intestine lengthens as it herniates and rotates in an anticlockwise direction around its vascular supply, the superior mesenteric artery (SMA), prior to returning to the body cavity ( Ch. 66 ). The final steps of differentiation and production of intestinal enzymes are completed at approximately 12 weeks of gestation.
Clinical anatomy
Anatomically and surgically, the small intestine is usually defined as three segments, the duodenum, jejunum and ileum, with an approximate total length of 6 metres. The duodenum is primarily retroperitoneal as it follows the head and body of the pancreas. While the ligament of Treitz defines the origin of the jejunum, there are no surgical landmarks that identify its transition to the ileum; it is typically defined as being composed of the distal 59% of the small intestine. The jejunal and ileal segments are free within the abdominal cavity, tethered only to the retroperitoneum proximally by the ligament of Treitz, and distally at the caecum where the ileum terminates at the ileocaecal valve.
Histological variation in the four layers of the small intestinal wall (mucosa, submucosa, muscularis externa and adventitia/serosa) is part of what defines the three intestinal segments beyond the traditional surgical landmarks ( Fig. 59.1 ). The jejunum may be distinguished by its more prominent mucosal folding and thicker lumen. The mucosa is subdivided into a lining epithelium, an underlying lamina propria (a layer of loose connective tissue, where many of the glands are also found) and a thin layer of smooth muscle, the muscularis mucosae. The absorptive cells, enterocytes, generate microvilli that appear as a striated brush border on the surface of the villi and help to increase the effective surface of the small intestine by 600-fold. At least seven different cell types (including enterocytes, goblet cells, Paneth cells, enteroendocrine cells, tuft cells and lymphocytes) populate this layer and are regenerated from the crypt base as part of an ongoing process of proliferation, differentiation and recycling. The submucosa is a connective tissue layer that provides the dense network of arteries and lymphatics that is key for the movement of nutrients through the small intestine. It is also considered the strongest layer of the intestinal wall and, as such, is critical for suturing and manipulating intestine for anastomotic procedures.
Chyme is moved along the small bowel by peristalsis, coordinated by the myenteric plexus within the submucosa and primarily generated by the muscularis propria. Distension of the stomach initiates gastroenteric and gastroileal reflexes that increase glandular secretion and peristaltic activity in the duodenum and small bowel, and relax the ileocaecal junction. The serosal surface, or adventitia, serves as the outer connective tissue envelope of the small intestine.
Vascular supply, lymphatic drainage and innervation
The gastroduodenal artery (usually a branch of the common hepatic artery) forms a plexus of pancreaticoduodenal vessels that is most prominent along its interface with the pancreas; it usually gives off retroduodenal branches that supply the first part and proximal portion of the second part of the duodenum, and a supraduodenal artery that supplies the anterosuperior part of the proximal duodenum. However, the vascular supply of the small intestine is derived primarily from the SMA. Given the importance of blood supply to anastomotic viability, it is very important for the surgeon to understand the anatomical variations that may be encountered during operations on the small intestine. Most variability is observed in the second- and third-order branches of the SMA. The first-order branches typically involve the right colic artery and the ileocolic trunk, which supply the jejunum and terminal ileum ( Fig. 59.2 ). The vascular arcades of the ileal and jejunal segments typically exhibit two orders of branching; the distal perforators enter the small intestine less than a centimetre apart, providing a highly redundant and intricate vascular supply with significant intramural anastomoses. The density of this blood supply appears to be more pronounced in the jejunum, where branched vasa recta and an increased density of plicae circulares (producing thicker jejunal walls) differentiate the jejunum from the ileum. Venous drainage of the small intestine is primarily derived from the superior mesenteric vein (SMV), which develops within the mesentery, ascends along the right side of the arterial arcade, and typically joins the splenic vein ultimately, to form the portal vein just deep to the neck of the pancreas.
The lymphatic system of the small intestine regulates tissue fluid homeostasis, participates in immune surveillance, and transports dietary fat and fat-soluble vitamins from the gut lumen. It is organized into two networks. Lacteals from the villi drain into a plexus of lymphatics in the submucosa and are joined by vessels from lymph spaces at the bases of solitary lymphoid follicles. A coarse plexus of lymphatics also runs in the muscularis externa between the two muscle layers; submucosal and muscular networks share few connections but both communicate freely with larger, valved collecting lymphatics at the mesenteric border of the small intestine.
Duodenal lymphatics run to superior and inferior pancreaticoduodenal lymph nodes, and from there to supra- and infrapyloric, hepatoduodenal, common hepatic, coeliac and superior mesenteric nodes. Jejunal and ileal lymphatics drain from mesenteric nodes initially to superior mesenteric nodes around the root of the superior mesenteric artery. Individual segments of small bowel have a relatively wide field of lymphatic drainage, which makes radical surgical resection of draining lymph nodes difficult if the blood supply to the remaining unaffected small bowel is to be preserved.
The innervation of the small intestine includes the enteric nervous system (made up of motor neurones, intrinsic sensory neurones, and interneurones lying within the wall of the gut), the autonomic nervous system (sympathetic and parasympathetic innervation) and visceral (sensory) afferents. The coeliac plexus surrounds the coeliac trunk and the superior mesenteric artery. It contains the coeliac ganglia, which lie on either side of the coeliac trunk, medial to the adrenal (suprarenal) glands and anterior to the crura of the respiratory diaphragm. It receives a significant input from the thoracic splanchnic nerves, which carry preganglionic sympathetic fibres from the lower seven thoracic sympathetic ganglia. The greater splanchnic nerves are derived from the fifth to the ninth or tenth thoracic ganglia. They descend obliquely in the posterior mediastinum, give off branches to the descending aorta, perforate the crura of the diaphragm and end in the coeliac ganglia. The lesser splanchnic nerves are usually formed by the rami of the ninth and tenth thoracic ganglia (occasionally by the tenth and eleventh ganglia). They pierce the diaphragm with the greater splanchnic nerves and end on the aorticorenal ganglia. When present, the least splanchnic nerves originate from the lowest thoracic ganglia and enter the abdomen with the sympathetic trunks before ending in the renal plexuses. Cadaveric studies have confirmed that the greater splanchnic nerves are invariably present, the lesser splanchnic nerves are present in up to 90% of individuals, and the least splanchnic nerves occur in approximately 50% of individuals.
The duodenal wall is innervated by postganglionic sympathetic axons distributed via peri-arterial plexuses on the branches of the coeliac trunk and superior mesenteric artery. The jejunum and ileum are innervated by parasympathetic and sympathetic fibres via the superior mesenteric plexuses. The sympathetic nerves are vasoconstrictor to the vasculature and inhibitory to the musculature of the jejunum and ileum; sympathetic neurotransmitters also have an immunomodulatory role by influencing mucosa-associated lymphoid tissue. Preganglionic parasympathetic axons travel in the vagus nerves and are secretomotor to the mucosa and motor to the smooth muscle of the jejunum and ileum. Visceral afferents from the small bowel, conveying pain and other gut sensations, travel with the splanchnic and vagus nerves, predominantly the latter.
Congenital anomalies
While congenital anomalies may represent a relatively uncommon indication for small intestinal surgery overall, especially on adult patients, it is still important to be aware of the spectrum of anomalies, and how to address them at operation if they are unexpectedly encountered. The most common anomalies are those that involve non- or malrotation and incomplete fixation ( Ch. 66 ). Such congenital anomalies fall on a spectrum that ranges from complete to incomplete or reversed rotation. The central point is the ligament of Treitz, in that the duodenal–jejunal junction is fixed to the left of the spine, with a colonic attachment in the right lower quadrant. Under circumstances of non-rotation, the mesentery of the small intestine becomes a narrow pedicle, creating the potential for volvulus and obstruction, and potentially even strangulation. Other anomalies of the small intestine that the surgeon may encounter include a Meckel's diverticulum, duodenal webs or atresia, jejunal atresia and small bowel duplications. Congenital anomalies may not be recognized until later in life, when a patient presents with an unrelated problem, such as appendicitis. They may be identified preoperatively by imaging or at the time of laparotomy. Irrespective of aetiopathogenesis, the treatment is similar: the redundancy of the small intestine will usually permit resection of the defective segment and primary anastomosis.
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